Solvent vapor incineration apparatus of the type to which this invention relates is typically employed in connection with a web printing operation wherein combustible solvent vapors are given off from the web as it passes through a dryer enclosure just after being imprinted. Combustion or oxidation of such solvent vapors is necessary for the avoidance of air pollution and is also a safety measure, to prevent vapor concentrations in the dryer from reaching an explosive level. In the dryer, heated air is blown against the web to expedite the drying process, and exhaust air mixed with solvent vapors is drawn out of the dryer into a duct that conducts it to the incineration apparatus.
In the incineration apparatus, substantial energy efficiency is realized by effecting oxidation of the solvent vapors in a catalyst bed. In order for such catalytic incineration to take place, the gas stream of exhaust air and solvent vapors must be heated by means of a fuel burner before it is delivered to the catalyst bed, but the temperature to which the gas must be heated is well below the kindling point of most of the solvent vapors, although higher than the temperature of the air blown into the dryer. A significant portion of the heat generated by and for oxidation of the solvent vapors can be reclaimed from the hot gas issuing from the catalyst bed by passing that gas through a pair of heat exchangers, one for heating the incoming gas stream flowing towards the fuel burner and the other for heating air flowing towards the dryer.
It is well known that the temperature of the gas stream entering the catalyst bed must be so controlled as to be kept within certain limits. It must be high enough to ensure that complete oxidation of the solvent vapors will occur in the course of flow through the catalyst bed, but it should not be substantially higher, for otherwise there is a waste of fuel in the unnecessary heating of the gas stream before it enters the catalyst bed. In an extreme case a heat exchanger could be damaged by excessive temperature of gas issuing from the catalyst bed.
It is not unduly difficult to maintain a given gas stream temperature at the inlet to the catalyst bed. The temperature there depends upon the amount of heat energy in the gas stream leaving the dryer and the amounts of heat energy that are added to the gas stream at the heat exchanger and the fuel burner which preheat the gass for its delivery to the catalyst bed.
However, the problem that has heretofore confronted the art was not how to maintain a given temperature at the inlet to the catalyst bed, but what temperature to maintain there under any given conditions.
The optimum temperature to be maintained at the inlet to the catalyst bed is not a constant value but is a variable which depends upon the relative concentration of solvent vapors in the gas stream, and that concentration, in turn, is subject to very substantial and more or less unpredictable variation. By way of example, with a low solvent concentration in the gas stream, a temperature of 700.degree. F. at the inlet to the catalyst bed might result in maintenance through the whole bed of a temperature not much over 700.degree. F., due to the low heat release from oxidation of the small amount of solvent in the gas, but this temperature in the bed would be high enough to effect complete combustion of the solvent at a low concentration. However, if there were to be a substantial increase in solvent concentration without change in the 700.degree. F. inlet temperature, there would be a substantially greater heat release in the catalyst bed, due to the higher rate of combustion of solvent, and the temperature of the gas issuing from the catalyst bed would be much higher than necessary.
From the foregoing example it might seem as if the temperature of the gas stream at the inlet to the catalyst bed could be controlled by reference to the temperature of the gas at the outlet from the bed. However, if inlet temperature were controlled to maintain a constant outlet temperature set point, a set point suitable for a low solvent concentration would be too low for a higher solvent concentration. With a low solvent concentration the gas stream undergoes only a small temperature rise as it flows through the catalyst bed, and therefore if the outlet temperature set point were, for example, 720.degree. F., the temperature of a gas stream with low solvent vapor concentration would be close to the desired 700.degree.-720.degree. F. range during its entire flow through the catalyst bed. However, a gas stream with a substantially higher solvent concentration would undergo a much greater temperature rise in its flow through the catalyst bed, and maintenance of the 720.degree. F. outlet temperature would require the temperature at the inlet to be so much lower than 720.degree. F. that temperatures in the catalyst bed would not be high enough for proper incineration except in a zone very close to its outlet, with the result that there would be incomplete combustion of solvent vapors.
Controlling the temperature of the gas stream at the inlet to the catalyst bed by monitoring the temperature of the gas at some intermediate point in its flow through the bed is likewise impractical. The catalyst bed has a relatively short length in the direction of stream flow, but has a relatively large cross-section transverse to that direction so that the gas stream velocity through the bed is relatively low. The problem of suitably locating a monitoring sensor at a point between the inlet and the outlet of the catalyst bed would be complicated not only by criticality due to the short flow length but by the fact that gas temperature rises relatively rapidly just inside the inlet to the catalyst bed and then rises more slowly during the course of flow through the remainder of the bed. Furthermore, at any given distance from the inlet to the catalyst bed, temperature tends to vary from point to point around the bed, due to temperature distribution conditions that depend upon aging of the catalyst and flow conditions (turbulence) within the bed that vary with gas flow velocity.
The problem of controlling the temperature of the gas stream at the inlet to the catalyst bed is further complicated by the characteristics of the fan or exhaust blower by which the gas is drawn out of the dryer and propelled through the fuel burner and the catalyst bed. The centrifugal type of blower conventionally used for this purpose tends to pump air or other gas at a fixed volume flow rate, regardless of density of the gas, and therefore the mass flow rate through the blower (which can be expressed in pounds of gas per unit of time) varies with the temperature of the gas at the blower, decreasing as that temperature rises.
It has heretofore been conventional to locate the exhaust blower close to the dryer, to suck gas out of the dryer and blow it through the primary heat exchanger, the fuel burner and the catalyst bed. Since the gas in the dryer is at a relatively constant temperature, this location of the blower assured a fairly constant flow of air from the dryer. However, it is desirable that air mass flow rates through the dryer be increased at times of increased vapor concentration, to ensure that vapor concentration in the gas stream and in the vicinity of the dryer will be maintained well below the explosive level and to increase the difference in pressure between the atmosphere and the interior of the dryer (which is normally at subatmospheric pressure) to prevent any possibility of solvent vapors being belched out of the dryer enclosure.
If the exhaust fan were located downstream from the catalyst bed, changes in vapor concentration would bring about rather substantial changes in the rate at which gas would be drawn out of the dryer and through the bed, but such changes in rate of gas flow would be in the wrong direction. With higher vapor concentrations, the gas stream issuing from the catalyst bed is normally hotter than with low concentrations and therefore has less density; consequently, mass flow rate through a blower located downstream from the catalyst bed would decrease at just those times when it should be increased.